While lateral patterning of semiconductors is readily obtained through photolithography, there is a need for an improved method to vary the thickness or composition of layers along a device. A simple technique to controllably adjust the vertical dimension would be extremely useful for many optical devices. For example, in Vertical Cavity Surface-Emitting Lasers (VCSELs), or resonant cavity photodetectors, the thickness of the resonator controls the wavelength, and by varying this thickness along a device or in any array, multiple wavelength operation can be achieved.
The simplest method for varying the thickness along a wafer is by relying on growth-induced non-uniformities. This method has been used to yield multiple wavelength layer arrays and LEDs. A more advanced technique, selective area epitaxy, uses an oxide mask to shield part of the wafer during metalorganic chemical vapor deposition (MOCVD) growth and thus cause an increased growth rate on the un-masked portions. This method has been successfully used to form waveguide laser arrays and in a modified form used for vertical cavity lasers. Another technique in molecular beam epitaxy (MBE) uses local heating to decrease the growth rate to yield a somewhat controllable taper in the thickness. Masks have also been used in an MBE chamber to shield parts of the wafer during growth and give discrete changes in the thickness. Finally, repeated lithographic and etching steps have also been used for multiple wavelength arrays.